Dark Matter (DM) is a crucial component of the universe and, for calculations of the cosmic microwave background (CMB), DM is successfully modelled as a pressureless perfect fluid within General Relativity (GR). With data from Planck it becomes possible to test generalisations of this model, searching for DM properties beyond the pressureless perfect fluid and thereby testing the CDM paradigm itself. Although there is no unique way to generalise a pressureless perfect fluid, the Generalised Dark Matter (GDM) model has proven useful in CMB applications (Hu astro-ph/9801234, Kopp et al 1605.00649, Thomas et al 1601.05097). The 3 new parameters of the model describe DM as an imperfect fluid with pressure and shear viscosity. I present new constraints from Planck data for constant and generally time dependent GDM parameters, finding no evidence for DM properties beyond that of a pressureless perfect fluid.